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Some weigh less than pennies. Others have wing spans up to six feet. Bats are an incredibly diverse species (with 1,400 different known types, in fact) and are found on nearly every part of the planet (1). Associated with darkness, death, and the supernatural (2), they have been mythologized in folklore across human history, but their impact on our lives is far from fictitious. In fact, researchers think that the 2019-nCoV virusthat has since taken over the world in the form of a global pandemicmay have originated in bats (3).

Bats have been associated with some of the worst viruses in recent human memory, including SARS, MERS, Ebola, and Marburg (4), and it is no coincidence. In terms of carrying deadly zoonotic viruses (i.e. viruses that can be transferred to humans from animals), bats surpass their animal competition. They can host upwards of 60 zoonotic viruses, a figure rivaled only by rodents, another common primary viral host (5). The question follows: why do bats havewhat appears to beheightened viral breeding capabilities?

Most researchers have turned to bats’ immune systems to find answers. A recent eLife study from the University of California, Berkeley suggests that the viral immune responses of bats are so fierce that they drive viruses to replicate and evolve faster (4). As a result, bats are often reservoirs of rapidly evolving viruses, thereby increasing the likelihood that a dangerous strain emerges. This means that when the virus occasionally jumps from bats to other mammals that have weaker immune systems, disaster strikes. Worse, the increased virulence of these bat-bred viruses likely makes them able to infect humans at a surprising rate, despite not being closely related.

With a similar emphasis on the bat immune system, other researchers have explained the animal’s heightened viral-breeding capacity by focusing less on its fierce immune response to invaders and more on its ability to limit that very same immune response. In fact, the presence of foreign invaders is not always enough to cause disease. Rather, overly robust inflammatory responses to infection, perhaps counterintuitively, can lead to pathological damage, e.g. by mistaking the organism’s own cells and genetic material as foreign and attacking them (6). Bats, however, can weaken their own inflammatory responses by controlling certain antiviral immune pathways like the STING-interferon pathway (7). The ability to downregulate the immune response to the minimum necessary to fight invaders is especially crucial for the flying mammals. Flight is energy-demanding and can often destroy cells, releasing DNA into places where it should not be (8). While most mammals would mistake these pieces of DNA for viruses and launch taxing inflammatory responses, bats prevent overreactions that could potentially harm themselves. Thus, bats’ resulting resilience enables them to host and transmit viruses for a longer time than other common viral hosts.

Given this information about bats’ unique ability to host a wide variety of viruses due to their immune systems, it is important to consider how this attribute actually affects the risk of viral transmission to humans.

Currently, bats, as well as rodents, are already scrutinized as “high-risk viral reservoirs.” However, recent statistical research models from researchers at the University of Glasgow have argued that understanding virus biology (i.e. how it replicates) is potentially more important for determining disease risk than studying the characteristics of the viral reservoirs (e.g. bats) themselves (9). These models showed that regardless of characteristics like the ability to host many viruses, there was not a single group of animal hosts that substantially increased the risks that viruses posed to humans. As a result, the researchers behind these findings have encouraged a shift in research emphasis away from investigating what makes certain reservoirs inherently infectious and towards determining the transmission risk of specific viral traits and other factors that bring humans closer to animals.

That said, the debate on whether or not “high-risk viral reservoirs” are substantially more infectious than others is far from conclusive. In the absence of scientific consensus and given the practical limitations of time, attention, and resources, it seems that targeted surveillance of these animal groups will and should still be the norm.

Whether or not bats are ultimately vindicated or found guilty, the fact remains that simply understanding the origins of infectious disease will not be enough to entirely prevent future outbreaks. As humans continue to come into closer contact with “high-risk” animals over time (5), preemptive measures will inevitably need to be coupled with more effective responses. It is up to the political leaders, scientific institutions, and communities of tomorrow to learn from the mistakes of the COVID-19 pandemic and ensure that future responses are both decisive and well-informed.

 

References:

  1. 13 Awesome Facts About Bats. (2016, October 24). https://www.doi.gov/blog/13-facts-about-bats
  2. Bat Myths and Folktales from Around the World. (2019, October 31). #FolkloreThursday. https://folklorethursday.com/folktales/bats-in-mythology-and-folklore-around-the-world/
  3. Gorman, J. (2020b, June 1). U.S. and Chinese Scientists Trace Evolution of Coronaviruses in Bats. The New York Times. 
  4. Coronavirus outbreak raises question: Why are bat viruses so deadly? (2020, February 10). ScienceDaily. https://www.sciencedaily.com/releases/2020/02/200210144854.htm
  5. Sanicas, D. M. (2018, August 29). What Makes Bats The Perfect Hosts For So Many Viruses? Medium. https://healthcareinamerica.us/what-makes-bats-the-perfect-hosts-for-so-many-viruses-3274c019bb4d
  6. The secret to bats’ immunity. (2019, February 26). ScienceDaily. https://www.sciencedaily.com/releases/2019/02/190226112401.htm
  7. Xie, J., Li, Y., Shen, X., Goh, G., Zhu, Y., Cui, J., Wang, L.-F., Shi, Z.-L., & Zhou, P. (2018). Dampened STING-Dependent Interferon Activation in Bats. Cell Host & Microbe, 23(3), 297-301.e4. https://doi.org/10.1016/j.chom.2018.01.006
  8. Gorman, J. (2020a, January 28). How Do Bats Live With So Many Viruses? The New York Times. https://www.nytimes.com/2020/01/28/science/bats-coronavirus-Wuhan.html
  9. Watson, C. (2020). Bats are a key source of human viruses—But they’re not special. Nature. https://doi.org/10.1038/d41586-020-01096-z